The aio_return() system call returns the final status of the asynchronous I/O request associated with the structure pointed to by iocb.
The aio_return() system call should only be called once, to obtain the final status of an asynchronous I/O operation once it has completed
(aio_error(2) returns something other than EINPROGRESS).

RETURN VALUES

If the asynchronous I/O request has completed, the status is returned as described in read(2), write(2), or fsync(2). Otherwise,
aio_return() returns -1 and sets errno to indicate the error condition.

ERRORS

The aio_return() system call will fail if:
[EINVAL] The iocb argument does not reference a completed asynchronous I/O request.